There is reason to believe that the lung is responsive to the actions of several hormones, but inadequate information exists. We propose to demonstrate that the lung is responsive to the direct actions of hormones by measuring the presence of hormone receptors in the rat lung, and correlating the presence of receptor sites for a given hormone to a defined biochemical event in the lung. In order to assess the concentration and rate of synthesis of pulmonary surfactant, a workable radioimmunoassay will be developed.